The present invention relates to a lifting device which automatically disconnects the lifting cable from the bundle of lifted material when the lifting device engages the bundle of lifted material that has been placed in an at rest position and claims the benefit of United States provisional application 60/003,748, filed Sep. 14, 1995.
In the past, there has been a need for a simple and efficient means of lifting bar joist, wooden roof joists, trusses or other bar or tubular material to position these materials at a work site for proper installation. In accordance with the prior art, the preferred method and apparatus for lifting and for positioning hanging bar, joist, roof joists and other bar or tubular material at a work site for installation has been the utilization of a crane lifting cable secured to a headache ball having a lift hook member secured to the ball. The lift hook member generally includes a spring biased closure member which prevents dislodgement from the lift hook member of an open hook member which is used to engage the shackled ends of a lifting cable that is engaged with the truss or bar stock to lift and move the truss or bar stock to the desired installation position. However, safety regulations have precluded the use of open hook members to engage shackled ends of a lifting cable because of the danger of injury due to accidental disengagement of the shackled end of the lifting cable from the open hook member.
To overcome such unsafe prior art devices, it has been suggested to utilize spreader cables with choker members which are specially wrapped around the bar stock or truss material for lifting the same. The spreader cable has an eyelet or ring which is engaged with the lifting hook member having a spring bias closure which prevents accidental removal of the spreader cable from the lift hook member. However, such devices although being safe for use, are slow and inefficient and require the workers to walk out onto the bar, joist or truss material to unhook the cable, a time consuming operation and, sometimes, a dangerous operation.
Finally, it has been suggested to utilize an automatic hook disengaging device which includes an attachment portion which engages the lifting hook or is directly shackled to the crane lifting cable. The disengaging device is comprised of a first portion which engages the cables or lifting hook and a center portion which is pivotally mounted to the first portion and includes a hook member pivotally mounted to the lower end thereof. In operation, such automatic hook release structures require the engagement of a lifting cable with the hook member and wherein the subsequent lifting of the bar or truss material pivotally moves the center portion to a vertical substantially parallel configuration to close the jaw of the hook member. When the raised load is lowered into position, contact of the hook device with the lifted object results in the hook member being positioned perpendicular to the vertical, with the center portion positioned at an angle to the upper first portion. However, such devices must engage precisely the top portion of the bar or truss material to cause the hook member to be positioned perpendicular to the vertical to release the hook member from the lifting cable, a condition which is difficult to achieve. Moreover, oftentimes when such automatic hook devices are in the open position, a worker still has to climb onto the lifted bar, joist or truss material to remove the lifting cable from the hook. Finally, such devices are expensive and, accordingly, have only found limited acceptance in the marketplace.